System and method for radiation beam measurement normalization

ABSTRACT

A radiotherapy system includes a radiotherapy device, such as a LINAC, operable to direct a radiation beam from a head thereof during operation, a field detector positioned to be within the radiation beam during operation of the radiotherapy device and operable to generate a beam measurement signal, a reference detector positioned to be outside of the radiation beam during operation of the radiotherapy device and operable to generate a reference signal, and at least one computer in signal communication with the field detector and the reference detector and configured with software to normalize the beam measurement signal based on the reference signal and to output a normalized beam measurement.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. Non-ProvisionalPatent Application Ser. No. 13/875,840, filed on May 2, 2013, whichclaims the benefit of U.S. Provisional Application Ser. No. 61/641,511,filed on May 2, 2012, the contents of which applications are hereinincorporated by reference in their entirety.

FIELD OF THE INVENTION

The present invention relates to radiotherapy systems, and moreparticularly, to systems and methods for normalizing radiation beammeasurements to compensate for variations in radiation output.

BACKGROUND OF THE INVENTION

Linear accelerator (LINAC) relative dosimetry (beam scanning) involvesacquiring a measurement of the radiation beam with a field detector andtypically a reference detector. The measurement is acquired as twoparameters are varied: the field detector's position and time. Using themeasured signal from the field detector, a plot of the LINAC's radiationbeam intensity versus position within that beam is possible, with anexample shown in FIG. 1.

Beam scanning often requires normalizing the measured signal from afield detector by the measured signal from a reference detector. Thisoccurs because the output of the LINAC may change with time, sometimesabruptly. For example, an increase in the LINAC radiation dose rateduring a scan results in an increase in the field detector's measuredsignal. If only a field detector is used, the beam scan will show achange in the beam intensity at that point in space that corresponds towhen the LINAC's radiation output changed. This irregularity in themeasurement is not indicative of the actual relative dose intensity ofthe radiation beam. In FIG. 2, a repeat of the scan from FIG. 1 isshown, but in which the LINAC's dose rate changed (increased) during thescan. This change caused the marked change in the amplitude of thebeam's relative dose intensity.

Small changes in LINAC dose rate do not affect the radiation therapytreatment. The change in the dose rate effectively changes the amount oftime in which the radiation dose is delivered, which is not important tothe treatment parameters being measured. The more important parameterduring beam scanning is the relative dose intensity of the beam. Usingonly a field detector may misrepresent this characteristic of theLINAC's radiation beam, as in FIG. 2.

To correct for changes in a LINAC's dose rate, a reference detector isused to normalize dose rate changes. For example, referring to FIG. 3, aradiotherapy system 110 includes a reference detector 120 that isstationary and located in the radiation beam 124. Considering the aboveexample, when the LINAC 112 dose rate increased, it increases themeasured signal of the field detector 116, which would otherwisemisrepresent the relative dose intensity of the radiation beam 124 (asin FIG. 2). However, the increase in the field detector's measuredsignal should be matched by a proportional increase in the referencedetector's measured signal. A ratio of the field detector's measuredsignal to the reference detector's measured signal corrects for thechanged dose rate that occurred during a scan. The measurement wouldthen match that of FIG. 1, correctly representing the beam's relativedose intensity. While this approach to beam measurement normalization isuseful, further improvements are possible.

SUMMARY OF THE INVENTION

In view of the foregoing, it is an object of the present invention toprovide radiotherapy systems with improved systems and methods forradiation beam measurement normalization. According to an embodiment ofthe present invention, a radiotherapy system includes a radiotherapydevice, such as a LINAC, operable to direct a radiation beam from a headthereof during operation, a field detector positioned to be within theradiation beam during operation of the radiotherapy device and operableto generate a beam measurement signal, a reference detector positionedto be outside of the radiation beam during operation of the radiotherapydevice and operable to generate a reference signal, and at least onecomputer in signal communication with the field detector and thereference detector and configured with software to normalize the beammeasurement signal based on the reference signal and to output anormalized beam measurement.

According to a method aspect, a computer-based method for normalizing aradiation beam measurement includes using at least one computerconfigured with software to receive a beam measurement signal from afield detector positioned within the radiation beam, receive a referencesignal from a reference detector positioned outside of the radiationbeam, normalize the beam measurement signal based on the referencesignal, and output a normalized beam measurement.

These and other objects, aspects and advantages of the present inventionwill be better appreciated in view of the drawings and followingdetailed description of preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is graph of a normalized beam measurement;

FIG. 2 is a graph of a non-normalized beam measurement;

FIG. 3 is schematic view of a radiotherapy system configured for beammeasurement normalization; and

FIG. 4 is a schematic view of a radiotherapy system configured for beammeasurement normalization, according to an embodiment of the presentinvention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to FIG. 4, according to an embodiment of the presentinvention, a radiotherapy system 10 includes a radiotherapy device 12, abeam scanning system 14 receiving signals from field and referencedetectors 16, 20, and a computer 22 in signal communication with thedetectors 16, 20 via the beam scanning system 14. The radiotherapydevice 12 is operable to direct a radiation beam 24 from a head 26thereof, the operation also producing head scatter 30. The fielddetector 16 is arranged so as to be in the radiation beam 24 andoperable to generate a beam measurement signal, while the referencedetector 20 is positioned to be outside of the radiation beam andoperable to generate a reference signal based on the head scatter 30.The computer 22 is configured with software to normalize the beammeasurement signal based on the reference signal and to output anormalized beam measurement.

The radiotherapy device 12 is advantageously a linear accelerator(LINAC) used in connection with beam scanning during operation (i.e.,measuring the relative dose intensity). Generation and movement of theradiation beam and rotation of the head 26 are performed by a LINACcontroller 32 capable of sending and receiving control signals to theLINAC 12 to effect device operation.

The beam scanning system 14 includes an electrometer 34 connected to thefield and reference detectors 16, 20 by respective cables 36, 40. Theacquired electrical charge of the detectors 16, 20 is thereby suppliedthe electrometer 34 for measurement, with the measured values beingoutput to the computer 22. If it is necessary to provide a polarizingvoltage to the field and reference detectors 16, 20, the polarizingvoltage is provided via their respective cables 36, 40 and theelectrometer 34.

The field and reference detectors 16, 20 are both configured to detectionizing radiation which, as indicated above, will alter the acquiredcharge of the detectors. The field detector 16 is placed where it willbe in the radiation beam 24 emitted by the LINAC 12 during operation.The reference detector 20 is placed at location that will be outside ofthe radiation beam 24 during operation, but still exposed to headscatter 30; for instance, on top of the head 26. In a LINAC, the headscatter is produced by interactions between the LINAC-generated beam andcomponents of the LINAC head. Notably, the depicted beam 24 and headscatter 30 are provided for illustrative purposes, and not intended toreflect actual geometries)

As used herein, the term “computer” (e.g., the computer 22) is used tobroadly indicate at least one hardware processor capable of executingprogram instructions, machine-readable memory media for storing suchinstructions and related data used and/or generated during the operationthereof. Typically, a computer will also include one or moreinput/output devices, such as video displays, keyboards, pointingdevices and the like. A “computer” can encompass multiple, physicallydiscrete and separately housed processors, memory media and the likethat are in signal communication (for instance, via a network) toperform the enumerated functions. Additionally, the computer 22 can bestructurally and/or functionally integrated with some or all of the beamscanning system 14 and/or LINAC controller 32.

The computer 22 is configured with software to output a normalized beammeasurement based on the beam measurement signal and the referencesignal. In particular, the software directs the calculation of the ratioof the beam measurement signal from the field detector and the referencesignal from the reference detector. This effectively normalizes the beammeasurement since changes in dose rate will affect the numerator and thedenominator in the same proportion. This is because the dose rate of thebeam and the heat scatter rate are linearly proportional. Minor changesin beam shape, on the other hand, will not affect the head scatter rate.

The present invention can offer several advantages over normalizationthat requires locating the reference detector within the radiation beam.For example, accurate beam normalization is still possible even wherethe radiation beam size too small to accommodate both a field detectorand a reference detector. Also, it is not necessary to reposition thereference detector as the beam size changes. The reference detector canbe securely mounted in a fixed location, making unintentional movementof the reference detector less likely. Additionally, as mentioned above,the reference detector will not be sensitive to fluctuations in beamshape during scanning, which might otherwise affect the referencedetector independently of the field detector.

The above described embodiments are presented for illustrative andexemplary purposes; the present invention is not necessarily limitedthereto. Rather, those skilled in the art will appreciate that variousmodifications, as well as adaptations to particular circumstances, willfall within the scope of the invention herein shown and described and ofthe claims appended hereto.

What is claimed is:
 1. A radiotherapy system comprising: a radiotherapydevice operable to direct a radiation beam from a head thereof duringoperation; a field detector positioned to be within the radiation beamduring operation of the radiotherapy device and operable to generate abeam measurement signal; a reference detector positioned to be outsideof the radiation beam during operation of the radiotherapy device andoperable to generate a reference signal; and at least one computer insignal communication with the field detector and the reference detectorand configured with software to normalize the beam measurement signalbased on the reference signal and to output a normalized beammeasurement.
 2. The radiotherapy system of claim 1, wherein theradiotherapy device is a linear accelerator (LINAC).
 3. The radiotherapysystem of claim 2, wherein the normalized beam measurement output is ameasurement of a relative intensity of the radiation beam versusposition.
 4. The radiotherapy system of claim 2, wherein the referencedetector is positioned to measure head scatter from the LINAC.
 5. Theradiotherapy system of claim 1, wherein the radiotherapy device includesa movable gantry that carries the head, and the reference detector iscarried on the movable gantry so as to move therewith.
 6. Theradiotherapy system of claim 5, wherein the head directs the radiationbeam away from a first side of the movable gantry and the referencedetector is carried on a second side of the movable gantry opposite thefirst side.
 7. The radiotherapy system of claim 1, further comprising atleast one electrometer configured to measure the beam measurement andreference signals and output respective measured beam measurement andreference signals to the at least one computer.
 8. The radiotherapysystem of claim 7, wherein the electrometer is connected to the fielddetector and the reference detector by respective cables.
 9. Acomputer-based method for normalizing a radiation beam measurement, themethod comprising using at least one computer configured with softwareto: receive a beam measurement signal from a field detector positionedwithin the radiation beam; receive a reference signal from a referencedetector positioned outside of the radiation beam; normalize the beammeasurement signal based on the reference signal; and output anormalized beam measurement.
 10. The computer-based method of claim 9,further comprising positioning the reference detector to receive headscatter from a radiotherapy device.
 11. The computer-based method ofclaim 10, wherein the radiotherapy device is a linear accelerator(LINAC).
 12. The computer-based method of claim 11, wherein positioningthe reference detector includes positioning the reference detector on anopposite side of a gantry carrying a radiation beam-emitting head of theLINAC.
 13. The computer-based method of claim 11, wherein the normalizedbeam measurement is a measurement of a relative intensity of theradiation beam versus position.
 14. The computer-based method of claim10, further comprising connecting the reference detector to anelectrometer with a cable.
 15. A system for normalizing a radiation beammeasurement, the system comprising at least one computer configured withsoftware to: receive a beam measurement signal from a field detectorpositioned within the radiation beam; receive a reference signal from areference detector positioned outside of the radiation beam; normalizethe beam measurement signal based on the reference signal; and output anormalized beam measurement.
 16. The system of claim 15, wherein thenormalized beam measurement is a measurement of a relative intensity ofthe radiation beam versus position.